Many products (e.g., food and beverages) undergo pasteurization in order to reduce the number of microorganisms in the product. The process involves heating a filled and sealed container at an elevated temperature for a time period sufficient to a pasteurize the contents. Desirably, the physical stability of the bottle and the biological stability and flavor of the contents are minimally compromised, thereby increasing the shelf life.
For example, there are various organisms in beer that, while not pathological or dangerous to humans, can affect the taste and appearance of the beer if allowed to grow. Draft beer does not require pasteurization because it is kept refrigerated and consumed in a short period of time. However, beer packaged in glass bottles or metal cans is traditionally pasteurized to achieve a long shelf life. In a conventional pasteurization process, known as tunnel pasteurization, water is sprayed onto a series of closely spaced packages as they move on a conveyor through a pasteurization tunnel, the tunnel being divided into a series of zones which may include preheating, heating, holding and cooling zones. The temperature of the beer in the containers is progressively raised to a desired level, held at this level for a predetermined period of time, and then cooled before exiting the tunnel. Generally, in order to insure complete pasteurization, the temperature of the beer at the “cold spot” (one quarter inch from the bottom of the center of the can or bottle) must reach a temperature of at least 140° F. for a sufficient period of time to produce a cumulative heating profile (e.g., a specified number of pasteurization units (P.U.), generally defined as the amount of heat imparted into the product during the elevated temperature and time period. Because the temperature of the beer generally increases when progressing from the cold spot to the top of the package, it is desirable to pasteurize at the lowest possible cold spot temperature (above 140° F.) to avoid overheating (and thus deforming or degrading) the rest of the product and package. One example of a tunnel pasteurization process is described for example in U.S. Pat. No. 4,693,902 to Richmond et al., the contents of which are hereby incorporated by reference in their entirety.
Although products such as beer have historically been pasteurized in glass bottles, it would be desirable to use plastic containers, e.g., containers comprising polyethylene terephthalate (PET) homopolymer or copolymers, to take advantage of PET's lighter weight and shatter resistance. However, producing a pasteurizable plastic beer container that can withstand the pasteurization time/temperature profile and provide a desired shelf life, at a price that is commercially viable, has been a long-standing need in the industry based on numerous problems which must be overcome. In particular, the range of temperatures encountered during pasteurization will cause a typical plastic container to undergo permanent, uncontrolled deformation (also known as creep).
Deformation is undesirable not only from an aesthetic perspective, but because it results in a loss of carbonation pressure. The volume growth undergone by a plastic container during pasteurization produces a drop in the product fill line, which increases the head space and results in a drop in carbonation (CO2) pressure in the liquid. This drop in CO2 pressure reduces the overall shelf life because the filled and pasteurized container is effectively starting with a reduced carbonation pressure. In various applications, it would be desirable to provide a pasteurizable beer container having an initial carbonation pressure of 3.3 volumes of CO2 (where “volumes”=volume CO2 per volume water) and a shelf life of 16 weeks.
Accordingly, there remains a need to provide pressurized plastic containers that can withstand pasteurization with reduced deformation.